In one form of strain gage, the device is formed as a thin film structure on the inflectible body. In another form, the strain gage is bonded to the inflectible body.
A number of different structures have been developed for utilizing variable resistances for use as strain gages and the like. Illustratively, a variable resistance pad is disclosed in U.S. Pat. No. 3,657,692 of Hans H. Wormser, wherein a portion of highly conductive material partly surrounds a channel in which there is placed resistive material of a specified geometric shape, the combination of the materials forming the resistance pad.
A piezoresistive strain gage structure is shown in U.S. Pat. No. 3,624,714 of James E. Frassrand. The transducer includes an edge-supported flexible diaphragm with a semiconductor chip providing a bridge arrangement of the piezoresistive strain gage areas bonded to the inner surface to position active tension gage areas at the center of the diaphragm and active compression gage areas at the periphery thereof.
In one form of strain gage, resistances are connected in a Wheatstone bridge arrangement. Roscoe A. Ammon discloses, in U.S. Pat. No. 2,740,093, a meter tester utilizing a Wheatstone bridge for determining the current sensitivity and internal resistance of direct current instruments, such as voltmeters and ammeters. To compensate for deviations in the unknown resistance of the meter from normal or rated resistance, the disclosed bridge is provided with a variable resistance having a plurality of ganged taps.
It is further conventional in strain gages to use an elastically deformable body arranged to concentrate the strains in localized portions thereof and more specifically, to localize compressive and tensile strains in different portions defined by an outer surface of the body. One such structure is shown in U.S. Pat. No. 3,341,796 of Walter H. Eisele. In connection with such a structure, an arrangement of a Wheatstone bridge circuit such that two of the strain gages of the circuit are mounted to the surface of the body defined by the high compressive strain portion and two of the strain gages are mounted to the body surface defined by the portion having high tensile strain. It is desirable in such a structure to provide the strain gage resistors to be of equal value in the unstrained state, i.e. to satisfy the equation: EQU (R1+R2)/R2=(R4+R3)/R3.
It has been necessary to provide some balance adjustment in the manufacture of such strain gages as the film manufacturing techniques of photolithography and sputter deposition do not provide the necessary high accuracy. Conventionally, such adjustment has been effected by laser trimming, anodizing, air abrasive methods, etc. Such techniques, however, have been found to adversely affect the performance characteristics of the gage.
As it is desirable to form the gage depositions to provide as nearly as possible original balance, the actual imbalance may be slightly positive or negative. Further, installation of the sensor element into the gage body and breaking-in treatment may alter the imbalance in either direction so that it is necessary to provide the final correction in either direction of adjustment.
Further, it is desirable to construct the gage compactly so as to permit all portions thereof to be subject to the same operational thermal conditions.